1. Field of the Invention
The present invention relates to a linear motor, and more particularly to an improved linear motor that undergoes straight-line translation motion and is therefore useful as high-speed repeated positioning means.
2. Description of the Related Art
In the field of transfer apparatuses, linear motors have hitherto been used as drive sources in constant-speed positioning or high-speed positioning. Any linear motor comprises a stator and a movable member. The stator is composed of a hollow cylindrical member and a plurality of magnets arranged in row in the hollow cylindrical member. The movable member is arranged in the stator, facing the outer surface thereof. A technique of arranging all magnets at intervals K, thus not only reducing the cost and the weight, but also decreasing the velocity ripple, has been proposed (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2005-237165).
Another technique concerting linear motors has been proposed (see, for example, PCT National Publication No. 2005-525773). In this technique, material having a relative magnetic permeability larger than 2.0 is selected for the hollow cylindrical member (sleeve) containing the row of magnets, thereby increasing the thrust and wear resistance of the linear motor.
If the linear motor is used as drive source in transfer apparatuses, particularly high-speed transfer apparatuses, it is demanded that the motor thrust be increased as much as possible in order to shorten the transfer time.
In view of this, interval K at which the magnets are arranged as proposed in Publication No. 2005-237165 has been studied. The publication teaches that interval K should preferably be about 10% or less of the axial length L of the magnets, to attain a speed and thrust, both well comparable with those attained if any adjacent magnets are set in contact. The publication also describes that if interval K is about 30% or less of the axial length L of the magnets, the resulting motor thrust will be large enough, though the speed is somewhat low.
However, the control of interval K of the magnets can reduce the speed ripple, but is not proposed for increasing the thrust.
Generally, the higher the relative magnetic permeability of any material, the more readily the material will be corroded. Hence, the higher the relative magnetic permeability the hollow cylindrical member of the linear motor disclosed in PCT National Publication No. 2005-525773 has, the shorter the lifetime of the motor will be, and the shorter the lifetime of the transfer apparatus incorporating the linear motor, if the transfer apparatus is used outdoors and therefore likely contacts water. Consequently, the transfer apparatus will be degraded in reliability.